Having a standard for classes would be significant even if no browser implemented it.

Nearly every JavaScript ecosystem has a class pattern. Ember, Angular, YUI, React, MooTools etc. All of them are incompatible.

A spec gives us a common target to iterate toward, even if we need to get there via transpilers. Just like standardizing on Promises means I can consume jQuery's ajax method without learning a new callback syntax, ES6 classes will mean I don't need to learn a new API for extending a parent class in every framework.

The Ember community has had amazing success adopting ES6 modules despite there being no browser support at all. I expect any framework would have a similar success story with classes.